In various communications systems, carrier sensing alone or in combination with a randomized back-off transmission time technique is used, as a part of a process of determining when a device will transmit on a physical communications channel.
Consider, for example, 802.11 based systems. In a typical 802.11 based system, a broadcast packet is transmitted on a channel based on a DCF (distributed coordination function) mechanism. The system may include multiple nodes. Each node maintains a back-off counter used in determining when the node may transmit data within an overall time interval in which devices may transmit data signals to one another. The back-off counter is initialized to zero.
Each node that wishes to broadcast senses the channel and transmits if the channel is sensed to be idle for more than a duration known as DIFS (DCF Inter Frame Space) and the device's backoff counter is zero. After each transmission, the transmitting node picks a new back-off timer, e.g., using a pseudo random function to determine the backoff timer value. If the timer expires before the next packet arrives for transmission, the device can transmit after sensing the channel to be idle for a DIFS duration assuming the last transmission in the system was successful. If the last transmission in the system was unsuccessful, the device needs to wait for EIFS (Extended Inter Frame Space). Whether a DIFS or EIFS is to be used can be determined from whether an acknowledgment is detected following a transmission.
In the event that a packet is waiting for transmission and the backoff counter is zero, but the carrier sensing detects that the carrier is occupied, the device will pick a second backoff timer value and transmit at the expiration of the second backoff timer value. Decrementing of the backoff timer value is based on sensing that the channel is unoccupied for a period of time with the amount the backoff time is decremented being a function of the amount of time the channel remains unoccupied from the point in time the backoff value was selected.
Typical drawbacks with the carrier sensing and back off scheme are that all the waiting nodes with zero back-off counter try to transmit at the same time when the channel becomes idle. This can occur when two transmitters choose the same back-off. Even if all waiting nodes have non-zero random back-off, the probability that 2 transmitters transmit at the same time is high when the node density is high given that there is a limited number of back-off values that may be selected. Note that the spatial configuration of concurrent transmitters in most ad hoc systems are not controlled by any protocol, i.e., the locations of the colliding transmitters can be arbitrary. This further leads to poor performance in receiving the broadcasting messages, especially in a dense deployment.
Collisions can be particularly problematic in systems where it is desirable and/or important to send small amounts of data on a somewhat regular basis. Collisions and the corresponding possible loss of data or delays related to re-transmission can cause what are intended to be regular transmissions appearing irregular in terms of spacing and/or can reduce the ability of devices to communicate at relatively uniform time intervals in a reliable manner.
This can be of particular concern in systems such as motor vehicle and/or other control systems, where it is important to provide frequent and/or periodic or semi-periodic data transmissions, which may be relied upon to avoid vehicle collisions and/other types of accidents.
In view of the above discussion it should be appreciated that there is a need for improved methods of controlling when and/or how devices contend for transmission resources. It would be desirable that at least some methods and apparatus reduced the probability of collisions in systems which repeatedly transmit, e.g., broadcast, small amounts of data on a periodic or semi-periodic basis.